Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. The appended claims are not intended to be limited, however, to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing or for sensing electrical signals produced by the heart, or for both pacing and sensing, in which case, a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled conductor surrounded by an insulating tube or sheath couples the connector pin at the proximal end and the electrode at the distal end.
To prevent displacement or dislodgment of the electrode and to maintain the necessary stable electrical contact between the lead tip and the endocardial tissue, the electrode must be firmly anchored relative to the tissue. To achieve this, the electrode of one known type of lead comprises a pointed helix adapted to be screwed into the heart tissue to be stimulated. Rotational torque applied to the connector pin at the proximal end of the lead is transmitted via the flexible, coiled conductor to the helical electrode which is thereby screwed into the heart tissue. In this fashion, the position of the electrode tip is sought to be mechanically stabilized by positively anchoring the tip so that it remains securely in place during the lifetime of the implant. Removal of the screw-in electrode from the endocardium can be effected by counterrotation of the connector pin. Thus, in a rotatable pin, screw-in lead, the conductor coil is used not only as a conductor for electrically coupling the connector pin and the helix electrode, but also as a tool for extending or retracting the helix electrode relative to the distal tip of the lead during lead myocardium fixation by rotating the connector pin.
One problem associated with rotating pin, screw-in type leads is the inability to easily verify the degree of extension of the helix electrode relative to the lead tip especially where the distal end portion of the lead assembly comprises an insulating tubular housing of polyurethane or the like that is not readily visible on a fluoroscope. Present approaches to helix extension verification are not altogether satisfactory. For example, U.S. Pat. Nos. 4,953,564; 4,972,848; and 5,002,067 disclose screw-in type cardiac pacing leads in which the helix electrode is secured to the coiled conductor by means of a crimping sleeve. A narrow, radiopaque indicator ring is disposed within the lumen of the electrode head behind the lead tip. According to the aforementioned patents, by fluoroscopically observing the distance between the crimping sleeve and the indicator ring, the distance that the helix has been extended can be determined. The separation between the crimping sleeve and the indicator ring cannot always be readily ascertained, however, because of the presence in the viewing area of other metallic elements, such as the crimping core and coiled conductor, which tend to obscure the relative positions of the sleeve and ring. This is especially a problem when the pacing lead tip is being viewed at an angle relative to the plane of the fluoroscopic image. Moreover, even under the best fluoroscopic viewing conditions, because the pacing leads of the aforementioned patents furnish only an indication of the distance between the crimping sleeve and a ring that is positioned behind the lead tip, they do not provide a direct indication of the extension of the helix relative to the pacing lead tip.
Accordingly, it is an object of the invention to provide a rotatable pin, screw-in type lead assembly, for use with an implantable medical device that permits direct and rapid fluoroscopic verification of the degree of helix electrode extension from the distal tip of the lead assembly.
Another problem associated with the type of leads under consideration is that despite the anchoring provided by the helix electrode, the position of the tips of screw-in type leads relative to the heart tissue is often unstable. This mechanical instability can produce excessive trauma and inflammatory tissue reaction resulting in an increase in the stimulation threshold of the heart.
It is thus another object of the invention to provide a rotatable pin, screw-in type lead assembly, for use with an implantable medical device, having a tip structure that promotes rapid lead tip stabilization after fixation so as to reduce myocardium damage and the associated inflammatory response, and the associated peak and chronic stimulation thresholds.
In screw-in type leads, a seal is typically provided between the outer insulating tube and the helix electrode (or the shaft carrying the helix electrode) to prevent blood or other bodily fluids from entering the lead body. The friction between the helix electrode or shaft and the seal is a factor determining the torque and therefore the number of turns required to fully extend and retract the helix electrode during lead fixation. It is desirable to minimize such friction and thus the number of revolutions of the lead connector pin required to fully extend or retract the helix electrode so that lead fixation can be effected both expeditiously and with minimum displacement of the tip relative to the fixation location. Presently available screw-in type leads, however, often require an excessive number of revolutions of the lead connector pin to extend and retract the helix as a result of high frictional forces between the helix and the fluidic seal.
Thus, it is yet another object of the present invention to provide a screw-in lead assembly in which friction between the helix and fluidic seal, and hence the torque and number of connector pin turns required to extend and retract the helix electrode, is substantially reduced.